The present invention relates to user equipment (UE) synchronization to a base station. More specifically, the present invention relates to an improved initial cell search method and system.
FIG. 1 illustrates a wireless communication system. The communication system has a plurality of base stations 2l-2n (2). Each base station 2 communicates with user equipments (UEs) 4l-4n (4) within its operating area or cell 6l-6n (6).
When a UE 4 is first activated, it is unaware of its location and which base station 2 (or cell 6) to communicate. The process where the UE 4 determines the cell 4 to communicate with is referred to as “cell search.”
In typical code division multiple access (CDMA) communication systems, a multi-step process is used for cell search. For step one, each base station 2 transmits the same primary synchronization code (PSC) in a primary synchronization channel (PSCH). In a time division duplex (TDD) communication system using CDMA, the PSCH is one timeslot out of fifteen for case 1 cell search (as shown in FIG. 2a), such as slot 0 or in general K, or two timeslots for case 2 cell search (as shown in FIG. 2b), such as slots 0 or in general K and K+8 and 8. Each base station transmits the same PSC in the PSCH timeslot(s). To reduce interference between secondary synchronization codes (SSCs) used in step two, each PSC is transmitted at a different time offsets. The PSC offsets are at a set number of chips.
The UE 14 determines the base station 12 to be synchronized to by searching the PSCH for received PSCs, such as using a matched filter. An example of the results of such a search are shown in FIG. 4. As shown in FIG. 4, peaks 261-262 occur in the PSCH where there is a high correlation with the PSC code. Typically, the search results are accumulated over multiple frames to improve accuracy. Using the accumulated results, the PSC peak locations are determined in the PSCH.
Referring back to FIGS. 2a and 2b, along with each base station's transmitted PSC, each base station 12 also simultaneously transmits secondary synchronization codes (SSCs), such as three, for both TDD case 1 and case 2. The SSCs sent by each base station 14 are used to identify certain cell parameters, such as the code group and frame timing used by the cell. The UE 14 typically uses a correlator to detect the SSCs and the data modulated on them at each PSC peak identified in step I. The UE 14 to read the broadcast control channel. In TDD step III for both types I and II, typically, the UE 14 detects the midamble used in the broadcast channel and subsequently reads the broadcast channel.
A drawback of the initial cell search system described above is that the performance of the second step (SSC detection) is governed by the quality of the received signal which could result in false detections if this signal is of poor quality. In past systems, the second step, receives no benefit from successful execution of step 1.
Accordingly, there is a need for an initial cell search system wherein the second step's performance is not solely governed by the received input signal, providing more accurate SSC detection.